Conventionally, research and development on electrophotography have been made through various attempts and technical approaches. The electrophotographic method forms an image through a process including: charging the surface of a latent image bearing member (hereinafter may be referred to as an “electrophotographic photoconductor” or a “photoconductor”); exposing the charged surface thereof to light to thereby form a latent electrostatic image; developing the latent electrostatic image with a color toner to thereby form a toner image; transferring the toner image on a transfer target such as paper; and fixing the toner image with a heat roller.
Contact heating-type fixing methods such as hot roller fixing methods have been widely used as methods for fixing toner. The fixing device used in the hot roller fixing methods is equipped with a heating roller and a pressure roller. In the fixing device, a recording sheet that bears a toner image thereon is allowed to pass through the pressure contact area (nip area) between the heating roller and the pressure roller, melting the toner image to thereby fix on the recording medium.
Resins mainly used for toners are, for example, a vinyl polymerizable resin and a resin having a polyester skeleton. These resins are superior or inferior in terms of functional properties of toners such as flowability, transferability, chargeability, fixability and image qualities. Recently, both of the resins are used in combination, or a so-called hybrid resin having both the skeletons is used.
Known toner production methods include: conventional kneading/pulverizing methods; and so-called chemical toner methods including: suspension methods and emulsification methods using an organic solvent and an aqueous solvent; suspension polymerization methods where droplets of polymerizable monomers are controllably polymerized to directly obtain toner particles; and aggregation methods where emulsified fine particles are produced and aggregated to obtain toner particles. As the chemical toners, core-shell toners have already been known, which include a core formed of a resin advantageous for thermal fixation where the core is covered with resin particles advantageous for charging and heat resistance.
For example, there has been disclosed a latent electrostatic image developing toner including a core of polyester resin and a coating layer of vinyl resin where the coating layer is formed, on the surfaces of colored resin particles produced by the emulsification dispersion method, using resin particles produced by the emulsification polymerization method or the emulsification dispersion method using a surfactant (see PTL 1).
Also, core-shell toners have been known which use as a resin material a polyester resin advantageous for strength, heat resistance and fixability. For example, there has been known a method including: forming core particles through aggregation/salting-out of a polyester fine resin particle dispersion liquid using an aggregating salt; then additionally adding a polyester fine resin particle dispersion liquid thereto and form shells through aggregation/salting-out thereof using an aggregating salt similarly; and then fusing the shells (see PTL 2).
Also, there has been known a method where the core-shell structure is formed through a process including: dissolving a polyester resin in an organic solvent; subjecting the solution to phase-inversion emulsification to form fine resin particles; and aggregating the fine resin particles with the addition of an electrolyte (see PTL 3).
Furthermore, there has been disclosed a method where a latent electrostatic image developing toner is obtained through a process including: forming core particles through aggregation and/or fusion of at least fine resin particles and colorant fine particles dispersed in a dispersion liquid; adding a liquid containing fine resin particles dispersed therein to a liquid containing the core particles dispersed therein; and forming a coating layer through aggregation and/or fusion of the fine resin particles on the surfaces of the core particles (see PTL 4).
Many conventional core-shell toners have a toner interior (core) enveloped with a shell and are designed to achieve both heat resistant storageability and low-temperature fixability. In addition, they are designed to be improved in chargeability by using a highly functional resin in the shell, or by forming the shell in color toners to thereby reduce the effect of the colorant.
However, when a large amount of the shell is formed in the core-shell toners, the shell is removed from the toner surface and the removed shell adheres to, for example, a toner-regulating blade. Whereas when the amount of the shell is too small or insufficient, the effects of the shell are obtained to cause background smear. In addition, the external additives are considerably embedded in the toner particles after degradation, which makes the flowability thereof insufficient.